Refactor get_gateway, get_integration logic

[tuxcanfly]

Right now, get_gateway and get_integration are a bit magical. If we want foo backend, we're looking for app.gateways.foo.FooGateway in all installed apps. In addition to being inefficient, it has a chance of causing unintended consequences due to layout of other installed apps. The only benefit is that we allow users to define custom backends by following the above convention.

Instead, I propose we should have a setting similar to CACHES e.g MERCHANT_GATEWAYS with key being the gateway name and value being the dotted path to the gateway class. Then get_gateway could simply load the class pointed at by the path. This will allow more flexibility in creating and laying out custom backends.

Same goes for integrations.

[tuxcanfly]

Backward compat: we can have a default for MERCHANT_GATEWAYS that points to the existing classes, so I as far as I can see, this change will be backward compatible. We'll need to display a DeprecationWarning for custom gateways to notify them to update to the new way of adding custom gateways.